A previously proposed circuit for the differential amplification of two input signals comprises an operational amplifier with a particular arrangement of feedback and input resistors of which the relative values are such as to achieve a true differential output from the amplifier. In order to achieve a high input impedance for each signal, which is desirable for reasons to be described later herein, the input signals have to be applied to the previously proposed circuit via respective high input impedance unity gain amplifiers. Thus, altogether, three amplifiers are needed which not only incurs a cost penalty but also can limit the obtainable performance.
The 1986 Linear Applications Databook published by National Semiconductor Corporation shows, on page 90, a proposed circuit for a high input impedance instrumentation amplifier using only two operational amplifiers. The input signals are applied to respective ones of the non-inverting inputs of the two amplifiers; the output signal is taken from the output of one of the amplifiers; and first, second, third and fourth resistors are connected respectively between ground and the inverting input of the other amplifier, between the inverting input and the output of this other amplifier, between the output of this other amplifier and the inverting input of the one amplifier and between the inverting input and the output of the one amplifier. The first and fourth resistors are given equal values while the second and third resistors are also made equal to one another and the differential gain of the arrangement is then determined by the ratio of the first and second resistor values.